Connecting two (2) Cisco 3270 routers via their 9 pin serial ports.

Unanswered Question
Mar 19th, 2008

I have a question that I just cannot seem to get an answer to. How can I get two(2) Cisco 3270 ruggedized routers to communicate via their 9 pin serial interfaces? They are IP addressable but I need the cable type and possible pins-out for the interfaces to come up. Please help me if you can or simply inform me it is not possible.

I have this problem too.
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andrew.butterworth Thu, 03/20/2008 - 10:45

For this to work you would need to run HDLC or PPP over the console ports. Since the console ports only operate asynchronously then PPP over an Async interface would be your only option (i.e. interface Async x). I have just checked and you can't configure this on a 3550 so I assume this is the same on a 3750.

It is possible to use the Aux port on an IOS router for this purpose but not on an IOS switch.

So the short answer is this isn't possible...



phillip-wright Thu, 03/20/2008 - 11:10


Thanks for replying, you view is most apprecieated, but I noticed you wrote in your response about 3550 and 3750 devices. Did you mean to reference these switches as opposed to the 3270 router in question? I just need clarification if I am to close this matter.


Richard Burts Thu, 03/20/2008 - 11:17


I believe that Andy made the same mistake that I did in the beginning which was to assume that your reference to 3270 was a typo mistake and that you were referring to some other (more common) model of Cisco gear. I have found (to my surprise) that there is a 3270 router. I am looking but have not yet found information about its serial connections.

Do not close the matter yet.



andrew.butterworth Thu, 03/20/2008 - 14:16

Yes, sorry my bad, I assumed you meant 3750 Switches as I have never heard of 3270 routers before. I just had a quick look and can only see references to the two serial ports being V24/RS232 DCE interfaces. Apart from that though I can't seem to find anything.

I suggest you raise a TAC case and get one of the TAC engineers that has some detailed experience with these routers.

It looks a serious piece of equipment though, for use in real harsh environments...


phillip-wright Thu, 03/20/2008 - 14:25


Thanks again for your dilegent responses!! This is a tough one. in respect to dealing with TAC I did that and got nowhere. They could not find anybody who had even worked on these routers(strange). I know about rs232; but I have no knowledge of the cpe in wheather the SMIC's can autosense between DCE and DTE and if so does it require special pin-outs or connectors.

andrew.butterworth Thu, 03/20/2008 - 14:49

I managed to find this that states DCE or DTE amd Sync or Async operation on the SMIC's.

This is the best info I can find, however it doesn't detail the pinouts...

Sorry I can't be any more helpful. I think you have got to go back to Cisco or your provider and get them to find the information for you.


Richard Burts Fri, 03/21/2008 - 04:54


I am surprised at the difficulty in finding any specific information about these serial interfaces and the cables that they would use. One thing that you could clarify for us which might be helpful as we try to help you find a solution: how will the serial interfaces of these routers be connected? Will this be a back to back directly connected cable or will the router connect to some kind of modem or CSU/DSU?

The first link that Andy provides may be the most helpful. As far as DTE and DCE are concerned Every Cisco router that I have dealt with will automatically recognize and process whether it is acting as DCE or DTE based on the pins active in its connector and on what it is connected to. If the router is connected to a modem or CSU/DSU then the router is DTE. If the routers are connected back to back then one router will be DTE and the other router will be DCE. The router acting as DCE will usually need to be configured with clock rate.

Andy's link indicates support for multiple serial standards including: EIA/TIA-232, EIA/TIA-449, EIA/TIA-530, EIA/TIA-530A, EIA/TIA-X.21, or CCITT V.35. I am familiar with EIA/TIA-232 in a 9 pin connector. The pinout for that would be (note that the first pin # is for the 25 pin connector and the second pin # is the 9 pin connector):

Pins commonly used for RS-232/EIA-232 (serial):

DB-25 D-sub-9 Signal Direction Signal Name

1 x Protective Ground

2 3 DTE-to-DCE Transmitted Data

3 2 DCE-to-DTE Received Data

4 7 DTE-to-DCE Request To Send

5 8 DCE-to-DTE Clear To Send

6 6 DCE-to-DTE Data Set Ready

7 5 x Signal Ground

8 1 DCE-to-DTE Received Line Signal Detector (Carrier Detect)

20 4 DTE-to-DCE Data Terminal Ready

22 9 DCE-to-DTE Ring Indicator

If you want more details here is the link where I found it:

There may be implementations of the other standards in 9 pin connectors but I am not familiar with them.




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